The present invention relates to a method and apparatus for reducing particles in the exhaust gases of, in particular, auto-ignition type internal combustion engines.
DE 35 28 445 A1 describes a method and apparatus in which energy is fed to the particles by an electromagnetic field generated by a microwave source in order to burn the particles. It has been found, however, that it is almost impossible to feed sufficient energy via the electromagnetic field to the freely streaming particles in the exhaust gas in order to heat the particles to the temperature of combustion and thereby burn them.
An object of the present invention is to provide a method and apparatus such that sufficient energy is transmitted to the freely streaming particles in the exhaust gas stream by the electromagnetic field in order to bring the particles to the temperature of combustion and burn them.
This object has been achieved according to the present invention by agglomerating the particles before passing them through the microwave energy field.
When the exhaust gases are treated by the method according to the present invention, the particles have a significantly larger size upon reaching the electromagnetic field than when in the untreated state. The energy of the electromagnetic field can be applied with significantly greater ease to larger particles of this type than to smaller particles which, when in their untreated state, have a diameter of the order of 0.1 .mu.m and are thus too small to enable microwave energy to be absorbed to any appreciable degree.
In the case of the particle size of at least about 10 .mu.m, there is, on one hand, advantageous application of microwave energy and, on the other hand, particle enlargement does not even involve too much expenditure. This enlargement is effected most advantageously in an electric high-voltage field of an electrostatic filter.
By concentration of particles in the exhaust gas stream fed to the microwave energy field, application of the microwave energy to the particles is further improved because, when the extent of the microwave energy field is constant, the particle retention time in this field is increased or the microwave energy field can be further concentrated. At the same time it is further advantageous if the particle concentration and division of the exhaust gas stream occur downstream from particle enlargement.
An apparatus for implementing the method according to the present invention contemplates the particle enlargement occurring in the electrostatic filter and energy application for burning the particles occurring in the resonator. A particular apparatus for concentrating the particles comprises a resonator through which an exhaust gas stream passes and in which a microwave source generates an electromagnetic field, and an electrostatic filter generating a high-voltage field arranged in the exhaust gas stream upstream from the resonator.